Steven J. Knapp, age 9, of Victoria, British Columbia, Canada, for his question:

Were sloths once as big as elephants?

Of all the land dwelling animals that share our world, the elephant is the biggest. No doubt you have seen him on TV and compared his size with a human being. Maybe you have seen a real live animal at a zoo or circus. His immense size makes your eyes pop. It is hard to imagine a bigger animal but many of our whales are much, much larger than elephants.

The slow going sloth makes his home in the leafy forests of Central and South America. The biggest ones are no bigger than a sizeable dog and they spend most of their time hanging down by their four feet from the branch of a tree. Of course, we all enjoy those wonderful jokes about elephants who climb trees. But the notion of an elephant sized sloth hanging from a bough seems ridiculous. Nevertheless, half of this notion is true. The ancestors of our little sloths were even bigger than our modern elephants. That part of the story is true. But they did not hang from the boughs.

We know about these great grand daddy sloths because their fossil bones have been found buried in the ground. There were even a few hairs and wads of fossilized skin along with the bones. A million years ago, these giant sloths roamed around South America. Some of them lasted until the first American Indians wandered down there, several thousand years ago. Sloth bones and skin have been found among the debris around ancient human campfires. The fossils of bear sized sloths that lived about 2,000 years ago have been found in New Mexico and in the La Brea tar pits of California.

These big sloths of the past did not spend their lives in trees. They were ground sloths who walked around on the earth. The giant of the group was 20 feet long    much longer than a modern elephant. However his legs were shorter. His coat was long and shaggy and he had a long, wide, hairy tail. His sturdy bones were very thick and heavy and he weighed about five tons. This is one ton more than our Indian elephant and one ton less than our African elephant. Some experts think that he was sturdy enough to walk upright on his hind legs    but, of course he was too heavy for tree climbing.

When the bones of the giant sloth were found, naturally a giant type word was needed to name him. Scientists named him the megatherium    which means The Great Beast. Actually, megatherium was not a beastly sort of beast. For all his mighty size and strength, he did not hunt down other animals. Most experts think that he did not eat meat at all. The gentle giant was a vegetarian who dined on leafy greenery. Most likely he used his mighty muscles to gather foliage from the green tops. If he stood on his hind legs, he could rock a sizeable tree until it toppled, then eat the tender greenery. Elephants, as you know, use their trunks to topple trees for the same reason.

We know that megatherium enjoyed life in the forests of South America about a million or so years ago. We do not know how long he lasted or why life became too difficult for him to survive. He shared his world with several smaller sloth cousins, and some of these lasted until two or three thousand years ago. Nowadays the only remaining members of the family are the two toed sloth and the three toed sloth that live in Central and South America. These fellows are so slow and lazy that mossy greenery grows in their coats and butterflies nest in their shaggy hair.